The isomerization of normal paraffins of low molecular weight is of considerable interest in the petroleum industry, in view of the very high octane number of isoparaffins.
The conversion of C.sub.4 -C.sub.7 and mainly C.sub.5 -C.sub.6 n-paraffins to isoparaffins has the distinct advantage of producing a high-octane motor fuel. This process can be used advantageously for improving light gasoline cuts and particularly straight-run head fractions.
Three different types of isomerization processes exist:
low temperature processes (at about 20.degree.-130.degree. C.) using a Friedel and Crafts catalyst such as aluminum chloride, PA1 "medium" temperature processes (at about 150.degree. C.) using as catalyst a supported metal such as platinum on halogenated alumina, and PA1 high temperature processes (at 250.degree. C. and more), using zeolite carriers associated with a group VIII hydrogenating metal. PA1 deposition of metal on the modified zeolite, disclosed in most of the above-mentioned patents, PA1 deposition of one or more metals on an inert binder, for example, alumina, and physical admixture with the zeolite in protonic form (patents to MOBIL Company, U.S. Pat. No. 3,432,568 and U.S. Pat. No. 4,374,926 and patent to UOP Company: U.S. Pat. No. 3,632,835). PA1 the total Si/Al atomic ratios are determined by X fluorescence analysis, the sodium contents by atomic absorption, PA1 the mesh volume and the crystallinity are determined by X-ray diffraction, the sample preparation being similar to the operating mode of Standard ASTM D 3942 80 prescribed for faujasite, PA1 the benzene adsorption capacity of mordenite is determined by gravimetry. The sample is previously desorbed at 300.degree. C. under 10.sup.-4 Torr (133.32 10.sup.-4 Pa). The adsorption is then conducted at 30.degree. C. for 4 hours under a benzene pressure P of 28 Torr (3733 Pa), which corresponds to a P/Ps ratio of 0.25, Ps being the saturating vapor pressure at the temperature of the experiment. PA1 either directly impregnated with the group IVA metal according to the above-described methods and then respectively admixed with the mordenite and with the binder and shaped by any method known in the art, PA1 or admixed with mordenite and the binder respectively and then shaped. The group IVA metal will then be deposited on the shaped product by any one of the above-described methods.
Irrespective of the type of catalyst used, the isomerization reaction is generally accompanied with a more or less substantial cracking reaction, depending on the catalyst and on the operating conditions.
"High temperature" processes have been disclosed in a large number of patents within the last twenty years. Most of them use a more or less extensively modified zeolite, particularly a mordenite, generally in acid form, with or without hydrogenation promoters.
Examples of such patents are those of SHELL Company using catalysts comprising mordenites modified by particular processes: U.S. Pat. Nos. 2,181,928, 2,272,737; 3,190,939; 3,442,794; 3,475,345; 3,836,597; 3,842,114; 4,359,409; and 4,400,576.
Other examples are patents to ESSO Company using partially dealuminated mordenites, such as U.S. Pat. Nos. 3,480,539 and 3,506,400 and a patent to MOBIL Company, U.S. Pat. No. 3,551,353.
Two modes of metal deposition are considered:
Particular treatments of the zeolite such as fluorine, introduction were also disclosed in U.S. Pat. Nos. 3,932,554 and 3,413,370.
Mordenite is characterized by a Si/Al atomic ratio usually ranging from 4 to 6; its crystalline structure is constituted by linking of SiO.sub.4 and Al0 .sub.4 tetrahedrons, generating two types of channels: channels with a dodecagonal opening (contour with a 12 oxygens) and channels with a octogonal opening (contour with 8 oxygens).
Two types of mordenite exist which are distinguished from each other by their adsorption properties: the so-called wide-pore form, always synthetic, which adsorbs molecules such as benzene (kinetic diameter=6.6.times.10.sup.-10 m), and the so-called small-pore form, natural or synthetic, which only adsorbs molecules of a kinetic diameter lower than about 4.4.times.10.sup.-10 m. These mordenites are also distinguishable by their morphological differences: needles for the so-called small-pore mordenite, spherulites for the so-called wide-pore mordenite, and structural differences: presence or absence of defects. In the above-mentioned literature, the so-called wide-pore variety is used.